This invention relates to a piezoelectric actuator element, and more particularly to a laminate type piezoelectric actuator element used in a piezoelectric actuator for a printer or the like.
There has been conventionally known a laminate type of piezoelectric actuator element in which plural piezoelectric ceramic layers of lead zirconate titanate (PZT) group and internal electrode layers are alternately laminated in a longitudinal (laminating or expanding and contracting) direction, and positionally displaced due to a piezoelectric and electrostrictive longitudinal effect thereof when a voltage is applied to the internal electrode layers. Such a laminate piezoelectric actuator element, which is called a longitudinal effect type of laminate piezoelectric actuator element, is expanded and contracted in the longitudinal or laminating direction thereof in accordance with polarity of the applied voltage to the internal electrode layers, and thus this displacing motion of the element has been utilized in an actuator for a piezoelectric dot impact printer head or a VTR head, or an oscillator for an ultrasonic motor or the like.
Each piezoelectric ceramic layer of this type of piezoelectric actuator element generally has a thickness of approximately 100 microns, however, in accordance with requirement for diversification of the laminate piezoelectric actuator element, a laminate piezoelectric actuator element which has thinner piezoelectric ceramic layers of approximately several tens of microns in thickness has been recently proposed.
In general, insulation assurance between the internal electrode layers is decreased and migration more frequently occurs between the electrode layers when the thickness of each of the piezoelectric ceramic layer is decreased below several tens of microns in thickness. In order to prevent deterioration of the insulation and improve a moisture-resistant property of the piezoelectric actuator element, the laminate piezoelectric element has been provided with an internal electrode structure in which so-called piezoelectrically-inactive portion having no internal electrode layer is formed at the peripheral portion of each piezoelectric ceramic layer.
In order to perform a driving operation with a low voltage in such a laminate piezoelectric actuator element, it is required that each of the piezoelectric ceramic layers should be thinner in thickness and the number of laminating layers thereof increased. However, as each of the piezoelectric ceramic layers is thinner in thickness and the number of laminating layers thereof is increased, it frequently occurs that a degreasing member used in a degreasing process is damaged, and inhomogeneity of each piezoelectric ceramic layer occurs in a sintering process, so that reliability of the piezoelectric actuator element is decreased.
Further, in the laminate piezoelectric actuator element having the piezoelectrically-inactive portion, the piezoelectrically-inactive portion has a restrictive action on the displacing motion of the piezoelectric actuator element when a voltage is supplied, that is, the displacing motion of the piezoelectric actuator element is disturbed by a clamping action of the piezoelectrically-inactive portion, and thus there occurs a problem that displacement loss, inhomogeneous displacement distribution, and internal stress occur in the piezoelectric actuator element.